Aqueous peroxy carbamyl group containing polymer systems and methods of their production and use

ABSTRACT

High performance polymer systems based on water dispersed polyurethanes obtained from the reaction of NCO terminated prepolymers with a peroxide compound containing at least one active hydrogen atom, methods of making such polymer systems and their use as hydrolytically stable flexible substrate bonding agents are disclosed. Flexible packages, e.g., retortable laminate pouches, are formed by bonding together sections of films, foils or like flexible substrates using such adhesives.

REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of copending application Ser.No. 134,790, filed Dec. 18, 1987, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention broadly relates to polymer systems used in convertingprocesses, methods for their preparation, methods of using them andpackages formed therewith. More particularly, it concerns new waterborne, peroxy carbamyl group containing polymer systems, methods offorming them from polyurethane materials, and methods of using them,e.g., to make flexible packages from films, foils or like flexible sheetmaterials, particularly, retortable pouches and like flexible packages.

2. Description of the Prior Art

In the flexible packaging industry, many types of polymer systems areemployed, e.g., adhesives, coatings, primers, binders, inks, etc. Thisinvention provides new, improved polymer systems useable as adhesives,coatings, primers, binders, inks or the like in converting or otherprocesses.

A number of adhesive systems that are solvent-based and a select fewwater-based systems are being used in the production of laminates andother materials used in the packaging of food, non-food and medicalproducts. While solvent-based adhesives have dominated this market for anumber of years due to economical and ecological reasons, there is agrowing need for competitive, high performance water borne adhesivesystems.

Although a number of water-based adhesive systems claim to fulfill thevarious needs of the packaging industry, they all exhibit variousdisadvantages. This is due to the fact that the lamination process usedin making packaging laminates, so-called converting, is complicated andinvolves many requirements for any acceptable adhesive system. Thus,useable adhesive systems should be machinable, compatible with a widevariety of printing inks, fast drying at very high coating speeds anddevoid of foam or other undesirable problems. Further, the adhesivesystems should be capable of bonding a host of different type films,paper products and foil to one another, yet provide very good initialand final bonding of such materials to give clear laminated endproducts. Depending on the product that packages made for such laminatesare to hold, a number of other requirements must be addressed, e.g.,retortable packages (see below) used for food or medical products shouldbe able to withstand high retort temperatures and/or pressures.

A wide range of monolayer materials and multilayer structures are usedin the flexible packaging industry for the containment of food, medicaland other products. To meet the required compatibility of the packagingmaterials to the contents of the resulting packages, a variety ofdifferent flexible materials are used, e.g., papers, films, foils,metalized films and metalized papers. For the sake of brevity, all suchmaterials will be referred to herein as flexiblele substrates. Also, theterm flexible packages as used herein encompasses packages made entirelyof flexible materials, e.g., pouches, as well as semirigid containersthat lave at least one flexible body wall or lid.

One important class of flexible packages extensively used to containfood and medicinals are called retortable packages, i.e., packagescapable of withstanding thermal processing in a pressure autoclave attemperatures in excess of 100° C. (see "Retortable Flexible andSemirigid Packages", pps. 568-571, Willey Encyclopedia of PackagingTechnology by Marilyn Bakker).

Retortable packages must maintain their material integrity as well astheir required barrier properties during product-to-package handling,thermal processing and subsequent shipping and handling. In the case ofshelf stable food packages, the flexible substrate used must beretortable and still maintain extended barrier characteristics. Inmaking such retortable packages, flexible substrates have been sealedtogether by adhesives that are solvent-based systems. The processing ofsuch adhesives takes a number of days, e.g., up to 13, to complete. Inaddition to this drawback, the solvent system adhesives presentenvironmental and economic problems so that is an ever growing need foraqueous system adhesives capable of replacing solvent system adhesives.This invention helps to fulfill this need with new forms of polyurethaneaqueous dispersions.

Aqueous polyurethane systems that are ionomeric in nature are a knownclass of product, see U.S. Pat. Nos. 4,598,121; 4,540,633 and 4,240,942,Canadian Pat. No. 837,176, German Pat. No. AS2624442 and referencescited therein, the disclosures of which are incorporated herein byreference for their teachings concerning formation of polyurethaneprepolymers that are converted into an ionomer containing free NCOgroups which are then reacted with a diol or other difunctional reagentto give chain extended polymers. This invention uses this known priorart with a new combination of reagents to produce new forms ofpolyurethane aqueous dispersions that have been discovered to possessunique adhesive properties.

OBJECTS

A principal object of the invention is the provision of new aqueouspolymer systems that can be used as the base for a variety of productsused in converting operations, including adhesives, coatings, primers,binders and inks.

Further objects include the provision of:

1. Such aqueous polymer systems that are highly efficient for use with avariety of films, foils and paper products.

2. Water borne adhesive systems that can form adhesive bonds capable ofwithstanding severe conditions of temperature and pressure, e.g.,package retorting conditions.

3. Aqueous adhesives that require much shorter processing time thanknown solvent system adhesives currently in use.

4. Aqueous system polyurethane adhesives that exhibit excellent adhesionin a variety of film/film and film/foil, etc. lamination applications,particularly retortable laminates.

5. New methods for the production of aqueous system adhesives.

6. New forms of retortable and other flexible packages.

Other objects and further scope of applicability of the presentinvention will become apparent from the detailed description givenhereinafter; it should be understood, however, that the detaileddescription, while indicating preferred embodiments of the invention, isgiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

SUMMARY OF THE INVENTION

The objects are accomplished, in part, in accordance with the inventionby the provision of new aqueous polymer systems comprising an aqueousdispersion of a reaction product of an NCO terminated prepolymer with aperoxide compound containing at least one active hydrogen atom, e.g.,hydrogen peroxide.

Preferred polymer systems of the invention have a solids content ofbetween about 20 to 50% of the total dispersion or a solids content ofbetween about 25 to 65% containing about 5 to 15% epoxy resin or othersuitable resin or resins that can react with pendant carboxyl groups orother groups of the polyurethane.

In the foregoing paragraph, the percentages given are by weight and arethe percentage of the specified component based on the weight of thetotal dispersion. This statement is also to apply throughout thisspecification and claims unless otherwise specified.

The water dispersable prepolymers used in accordance with the inventionare obtained by methods disclosed in the aforementioned prior artreferences by reaction of polyisocyanates with polyols in a mole ratioas close to one as possible such that the average molecular weight ofthe final system is as high as possible.

The objects are further accomplished by the provision of new flexiblepackages, particularly retortable packages, comprising a joint between aplurality of sections of flexible substrate bonded together by an layerof adhesive comprising a polyurethane polymer obtained by reaction ofNCO groups in a prepolymer with a peroxide compound containing at leastone active hydrogen atom.

Additionally, the invention provides new methods of forming flexiblepackages which comprises (a) providing an aqueous system adhesivecomprising an aqueous dispersion of a polyurethane obtained by reactionof NCO groups of a prepolymer with a peroxide compound containing atleast one active hydrogen atom (b) providing at least one flexiblesubstrate, (c) applying a layer of the adhesive to at least one selectedsection of the substrate and (d) forming at least one joint between theselected section of the substrate and another section of the same ordifferent substrate by sandwiching the adhesive layer therebetween.

Further, it provides new methods of producing an aqueous system adhesivefor bonding together flexible substrates which comprises (1) preparing aprepolymer containing free NCO groups by reacting an aromatic, aliphaticor cycloaliphatic diisocyanate with a polyol and an ionic groupcontaining organic compound, (2) forming an ionomer by reacting theresulting prepolymer with a tertiary amine and (3) forming an aqueousdispersion of polyurethane by reacting the ionomer in the presence ofwater with one or more peroxide compounds containing at least one activehydrogen. The quantity of peroxide compound reacted with the ionomer ispreferably in excess of the stoichiometric requirement for totalreaction with the free NCO groups in the ionomer.

In preferred methods of the invention, the anionic group containingcompound is dimethylol propionic acid and the tertiary amine istriethylamine added in sufficient amount to neutralize the anionicgroups contained in the prepolymer. Also, in some embodiments, theprepolymer may be prepared in the presence of an inert organic solventin the amount of about 1 to 10% by weight based on the combined weightof the solvent and the prepolymer forming reagents.

In the polyurethane dispersion production methods of the prior art citedabove, the reaction of an amine R¹ NH₂ with an isocyanate RNCO, theresulting compound contains a urea linkage: ##STR1## or if the reactionis with a diamine H₂ N--R¹ --NH₂, two urea linkages (diurea): ##STR2##

However, when the reaction of the isocyanate is with a peroxidecontaining at least one active hydrogen atom, e.g., R² OOH where R² ispreferably C1-C10 alkyl, cycloalkyl or aryl, in accordance with theinvention, a peroxy carbamyl group compound is formed: ##STR3##

In the case of hydrogen peroxide, which has two active hydrogen atoms,peroxy dicarbamyl group compounds containing O,O-dicarbamyl peroxidelinkages are formed: ##STR4##

It has been discovered as a result of this invention that these peroxycarbamyl groups in aqueous polyurethane dispersions bestow some unusualadhesive properties on the dispersions. Thus, such dispersions exhibitcritically improved bonding to most film and foil combinations used inthe flexible packaging industry. Further, the resulting bonds areretained even under harsh testing environments used in retortableapplications.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

To produce an aqueous system adhesive of the invention, first aprepolymer based on a di/poly isocyanate, di/polyol and an ionic groupcontaining organic compound, particularly an ionic diol, is preparedusing reactant proportions so that the resulting prepolymer containsfree NCO groups. The di/polyfunctional isocyanate used is selected fromthe class of aromatic, aliphatic or cycloaliphatic diisocyanates,preferably those containing 6 to 15 carbon atoms. Examples ofcommercially available useable isocyanates include "Desmodur-W"(4,4'-methylene bisdicyclohexyl diisocyanate) and isophorenediisocyanate. Adducts of the class isocyanates with di/polyols describedbelow and oligomers of the class isocyanates can also be used.

The di/polyols used in the invention are selected from dihydroxycompounds of molecular weight between about 2000 and 3000. Such diolscan be condensation products of commercially available dicarboxylicacids and diols. Also, polyether diols based on ethylene or propyleneoxide can be used. Commercially available diols of this class includethe "Formerez" series and "ED" series of polyols marketed by WitcoChemical Co.

The ionic group containing compounds used are carboxylic or sulfonicacids containing up to 10 carbon atoms and two hydroxyl groups. Apreferred compound is dimethylolpropanoic acid. Other useable compoundsinclude 1,3 diamino-2-propanoic acid, and 1,3 diamino alkylene sulfonicacids. The prior art patents referenced above contain numerous examplesof polyisocyanates, polyols and methods of reacting them with waterdispersable groups.

Alternatively, polyols carrying pendant carboxyl groups can be used,U.S. Pat. No. 4,460,738 discloses methods of making such polyols andsuch disclosure is incorporated herein by reference. Since these polyolshave groups that can be converted into an ionic group, a separatereaction with a compound such as dimethylol propionic acid is notnecessary. Also, mixtures of polyols with pendant groups that areionizable and polyols that do not have such groups may be used.

A catalyst, e.g., dibutyltin dilaurate, is advantageously used informing the prepolymer. Also, small amounts, e.g., 1 to 10% by weight,of inert solvent may be used if viscosity of the prepolymer presentsprocessing problems, e.g., problems is disbursing the prepolymer inwater. Examples of useable solvents include N-methyl pyrrolidone,commercially available as "M-Pyrole" from BASF, 1,4-dioxane, DMF andethylene glycol dimethylether. Also useable are monomers or polymersthat do not participate in the reaction under the prevailing conditions.Examples of such materials acting as inert solvents are alkyl acrylate,alkyl methacrylates, vinyl group containing monomers and/or polymers.

The prepolymer is then converted into an ionomer by reaction with atertiary amine, typically a trialkylamine, e.g., triethylamine.Following this, the ionomer is slowly added with agitation to a watersolution of peroxide compound containing at least one active hydrogenatom to produce the final polyurethane aqueous dispersion.

Hydrogen peroxide is a preferred peroxide compound for use in producingthe new polymer systems of the invention. Other useable peroxidereagents containing one or more active hydrogen atoms include t-butylhydroperoxide, cumene hydroperoxide, tetralin hydroperoxide and similaralkyl, cycloalkyl and aryl mono or poly hydroperoxides. Mixtures of oneor more peroxide compounds may be used. Also, instead of using suchreagents per se, materials that give rise to such peroxides in anaqueous system may be used. For example, instead of using hydrogenperoxide pre se, alkali metal peroxides, e.g., NaOONa, KOOK, alkalineearth metal peroxides or the like may be used as the peroxide reagentprecursor.

The prepared dispersion per se may be used as a polymer system inaccordance with the invention or it may be compounded with up to 15% byweight, preferably 5 to 15%, of commercially available melamine resins,e.g., "Cymel 301" from American Cyanamid, or epoxy resin, e.g., "Epon828" from Shell Chemical Co. The dispersions may also be compounded withup to 15% by weight of vinyl-type compounds, e.g., vinyl acetate, methylacrylate, styrene, etc., and vinyl group containing polymers.

Typically, the new polymer systems of the invention, when used asadhesives, are applied, e.g, by use of a drawdown bar, to a film, foilor. other flexible substrate to give a coating weight of about 1 to 2pounds, especially 1-1.5#, per 3000 square yards of substrate surface.The coated substrate is dried to remove substantially all the water. Thetacky surface is then bonded to a second same or different substrate byapplication of pressure to give good contact.

EXAMPLE 1

Into a three neck flask provided with a thermometer, mechanical stirrerand drying tubes, was placed 63 grams of "Desmodur-W" diisocyanate(Mobay) 197 g of "ED-3000" diol (WitcO), a drop of dibutyltin dilaurate.Then, 20 ml of "M-pyrole" solvent were added. The contents were heatedto 80° C. for an hour and 17.2 g of dimethylol propionic acid were addedplus another drop of DBTDL, after which the heating was continued for5.5 hours. At the end of this reaction period, 12.9 g of triethylaminewere added and stirring was continued for 15 minutes to form theionomer.

The resulting ionomer was slowly added over a period of 15 minutes to avigorously stirred solution of 5.4 g of 30% hydrogen peroxide dissolvedin 500 ml of water. The stirring was continued for an hour to completethe formation of the polyurethane dispersion which was stored for 24hours before being tested for adhesive Properties. The resultingdispersion contained about 32% solids and had a pH of about 7.4.

EXAMPLE 2

The procedure of Example 1 was repeated except that "Formerez 55-56", a2000 molecular weight polyester diol, was used in the place of"ED-2000".

The dispersions of Examples 1 and 2 were mixed with 15% by weight ofepoxy resin ("Epon 828") containing a small amount of surfactant andcoated on a variety of substrates by means of a drawbar to give acoating weight of 1-1.5#/3000 sq.yd. The coated substrates were airdried using a hair dryer to remove all water. The resulting tackysurface of each specimen was then bonded to a second film or foil andhand-pressed using a 4.5# roller to give good contact. Then, theresulting laminates were heated in a commercial, temperature controlledhot plate device; ("Sentinel" marketed by Packaging Industries GroupInc.) for 0.5 sec. at 180 F at 100 psi. Finally, bond strengths weremeasured at the initial time, after 24 hrs. and after 3 days in an ovenat 130 F. Also, the laminates kept for 3 days were heated in anautoclave at 260 F. for 20 minutes at 23 psi. to simulate a retortableenvironment and then dried.

Bond strengths were measured using ASTM (American Standard TestingMethods) D1876 method for adhesion peel values. The resulting test dataare reported in Table I.

EXAMPLE 3

Example 1 was repeated, except instead of hydrogen peroxide, 12.8 g of70% t-butyl hydroperoxide was used.

                  TABLE I                                                         ______________________________________                                        Laminates                  72 Hrs. After                                      Exp.    I         II     Initial 130° F.                                                                      Retort                                 ______________________________________                                        1       Polyester Foil   300     D*    D*                                             Foil      PP*    350     D     D                                      2       Polyester Foil   300     D     D                                              Foil      PP     37S     D     D                                      ______________________________________                                         D* = destruct bond                                                            PP* = polypropylene.                                                     

Further, destruct bonds (one or both of the substrates destruct beforethe adhesive fails) have been obtained in 3 days when stored at 130° F.The same results have also been obtained for a 10-day storage at roomtemperature. With the SOlvent based adhesives that are used for retortapplications, the lamination of polyester/foil takes 3 days when storedat 130° F., while foil/polypropylene lamination. Storage of 10 days at130° F.

The data of Table I shows that when hydrogen peroxide is used inaccordance with the invention, the laminates withstand the retortabilitytest destructed before the adhesive fails, have been obtained in 3 dayswhen stored at 130° F. The same result has also been obtained with 10day storage at room temperature. With solvent system adhesives that areused for retort applications, the laminates of polyester/foil takes 3days when stored at 130° C., but the foil/pp laminations require storageof 10 days at 130° F. Thus, the aqueous system adhesives of theinvention save much time in required bond formation, avoid use of costlysolvents and the use of unreacted "free isocyanates" that is the keycomponent of the solvent system adhesives. Another major feature of thenew adhesives is their ability to form required bond strength towithstand retorting by simply being stored for 10 days at roomtemperature. This is particularly useful in connection with film thatcannot withstand 130° F. heating, e.g., polyethylene films used in snackpackaging. Typical bond strengths obtained with 10 days storage at roomtemperature with a number of films to polyethylene film laminations whenthe adhesive was the Example 1 disperson to which was added 15% "Epon828" epoxy resin (Shell Chemical Co.) are reported in Table II.

                  TABLE II                                                        ______________________________________                                                  Initial                                                                             20 Hrs.   72 Hrs. 1 Week                                      ______________________________________                                        PE/Polyethylene                                                                           250     500       950   D                                         PE/polypropylene                                                                          200     525       675   D                                         PE/polyester                                                                              275     450       D     D                                         PE/nylon    300     500       650   D                                         PE/metal foil                                                                             325     500       625   D                                         ______________________________________                                         PE = Polyethylene.                                                       

Table III shows bond strength obtained using the product of Example 3 asthe adhesive system.

                  TABLE III                                                       ______________________________________                                                  Initial                                                                             24 Hrs.   72 Hrs. 1 Week                                      ______________________________________                                        Polypropylene/                                                                Polypropylene                                                                             250     400       600   D                                         Polyester/Foil                                                                            250     300       350   D                                         ______________________________________                                    

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. An aqueous polymersystem comprising an aqueous dispersion of a polyurethane containingperoxy carbamyl groups.
 2. An aqueous adhesive system for bondingtogether flexible substrates comprising the polymer system of claim 1.3. An aqueous adhesive system for bonding together flexible substratescomprising an aqueous dispersion of a polyurethane containingO,O-dicarbamyl peroxide linkages.
 4. An aqueous polymer systemcomprising an aqueous dispersion of a polyurethane reaction product ofan NCO terminated prepolymer and a peroxide containing at least oneactive hydrogen atom.
 5. A polymer system of claim 4 wherein saidperoxide is hydrogen peroxide and said reaction product containsO,O-dicarbamyl peroxide linkages.
 6. A method of producing an aqueoussystem adhesive for bonding together flexible substrates whichcomprises:preparing a prepolymer containing free NCO groups by reactinga 6 to 15 carbon atom aromatic, aliphatic or cycloalphatic diisocyanatewith a diol of molecular weight between about 2000 to 3000 and ananionic group containing organic compound, forming an ionomer byreacting the resulting prepolymer with a tertiary amine, and forming anaqueous dispersion of peroxy carbamyl group containing polyurethane byreacting said ionomer in the presence of water with a peroxide compoundcontaining at least one active hydrogen atom.
 7. The method of claim 6wherein said peroxide compound is hydrogen peroxide and said peroxycarbamyl groups are O,O-dicarbamyl peroxide linkages.
 8. The method ofclaim 7 wherein the quantity of hydrogen peroxide reacted with saidionomer is in excess of the stoichiometric requirement for totalreaction with the free NCO groups in said ionomer.
 9. The method ofclaim 8 wherein said ionic group containing compound is dimethylolpropionic acid and said tertiary amine is triethylamine added insufficient amount to neutralize the ionic groups contained in saidprepolymer.
 10. The method of claim 8 wherein said prepolymer isprepared in the presence of an inert organic solvent or diluent in theamount of about 1 to 10% by weight based on the combined weight of saidsolvent and said prepolymer forming reagents.
 11. The method of claim 10wherein said solvent is N-methylpyrrolidone.
 12. An aqueous systemadhesive prepared by the method of claim 8 having a solids content ofbetween about 20 to 50%.
 13. An aqueous system adhesive having a solidscontent of between about 25 to 75% containing about 5 to 25% melamineresin, epoxy resin or vinyl resin added to an aqueous dispersion ofperoxy carbamyl group containing polyurethane prepared by the method ofclaim
 8. 14. An aqueous system adhesive for bonding together flexible,non-porous film or foil comprising an aqueous dispersion of about 20 to50% by weight of the reaction product of (1) a prepolymer containingfree NCO groups formed by reacting a 6 to 15 carbon atoms aliphatic orcycloalphatic diisocyanate with a diol of molecular weight between about2000 to 3000 and an ionic group containing organic compound and (2) oneor more peroxide compounds containing at least one active hydrogen atom.15. The adhesive of claim 14 wherein said peroxide compound is hydrogenperoxide.
 16. The adhesive of claim 14 wherein said peroxide compound isan organic hydroperoxide.
 17. The adhesive of claim 14 wherein saidperoxide compound is an alkyl hydroperoxide.
 18. The adhesive of claim14 wherein said peroxide compound is supplied by a precursor capable offorming a peroxide compound containing at least one active hydrogen atomin the reaction system.
 19. The adhesive of claim 14 wherein saidperoxide compound is a mixture of peroxide compounds containing at leastone active hydrogen atom.